1. Field of the Invention
The present invention relates to a multi-hop network system including communication devices, a server that manages the communication devices, and a path notification method in the multi-hop network system.
2. Description of the Related Art
For mesh protocol technology in which multi-hop communication is performed via base stations to thereby attempt to enlarge a communication area, a communication scheme using a frame extension method or an extended field in standards such as IEEE 802.11s has been discussed (For example, see Hidenori Aoki, Shinji Takeda, Kengo Yagyuu, Akira Yamada, “IEEE 802.11s Wireless LAN Mesh Network Technology”, NTT DoCoMo Technical Journal, NTT DOCOMO, INC., Jul. 1, 2006, Vol. 14, No. 2, p. 14-22).
Moreover, there has been known a multi-hop network (or an ad-hoc network or an MANET (Mobile Ad-hoc Network)) used in a multi-hop communication scheme that enables packet transmission in a bucket brigade fashion by multi-hopping packet data among communication devices. For a routing protocol (a path selection/control protocol) used in this multi-hop network, routing schemes have been proposed/published in MANET working group of IETF (Internet Engineering Task Force) or IEEE 802.11s, and standardization thereof has progressed.
However, the above described techniques include the following problems.
As a first problem, in the multi-hop network, the larger the scale of the network, the higher is the performance that is required of the communication devices belonging to this network. Communication traffic increases along with an increase in the number of control packets transmitted and received in the entire network for routing control or the like, and an increase in the number of the communication devices. Thereby, congestion and transmission delay increase. Moreover, power consumption in the entire network also increases. This is due to the following reasons.
In the multi-hop network, it is necessary to create a communication path table indicating a routing path through which the packet data reaches a final destination. In order to create this communication path table, in a general routing protocol, the communication device first transmits a broadcast packet for path search. When receiving the transmitted broadcast packet, the communication device retransmits this broadcast packet in order to expand a path search range. In this way, a target packet transfer destination is found and a communication path table is created by propagating the broadcast packet for the path search, throughout the network.
In this way, in a large-scale network, according to the propagation of the broadcast packet throughout the network, the amount of packets transmitted for the path search increases, and a congestion and the transmission delay occurs in the network along with an increase in the number of transmitted and received packets.
Moreover, path information obtained by each communication device needs to be retained and updated.
Furthermore, a communication device with a low calculation capability includes a limited relay capability for the broadcast packet for path retrieval, or a limited relay capability for the transmitted packet. Thus, this limited relay capability may cause a transmission delay, or may decrease throughput of the entire network.
Due to these reasons, high calculation performance and abundant resources are required for the communication devices belonging to the large-scale multi-hop network. If each communication device performs processing depending on the requirement, power consumption in each communication device increases, which causes power consumption in the entire network to increase as well.
Moreover, now, proposed routing protocols include, as schemes applicable to the multi-hop network, schemes such as an AODV (Ad-hoc On Demand Distance Vector) protocol and an OLSR (Optimized Link State Routing), which have been discussed in MANET (Mobile Ad-hoc Network)-WG (Working Group). As a second problem, all of these schemes have to update the path information by periodical path check. This is due to the following reason.
In these protocols, it is assumed that the communication device moves and is powered OFF/ON, and thus periodical update of the path information is essential. This is because, when the communication device moves, the same ID (Identification) (for example, an MAC [Media Access Control] address) exists in locations, and thus routing cannot be performed.
As a third problem, in the multi-hop network, when the number of hops increases, that is, when the number of the communication devices to be passed through (the number of relay nodes) increases, communication quality degrades, and quality degradation such as a decrease in throughput occurs.
The reason is as follows. Since the number of times of transmission and reception of the packets increases along with an increase in the number of the relay nodes, the probability of packet discard increases. Also, transfer delay of the packet data increases due to process delay for the number of the relay nodes. Thus, in a protocol such as TCP (Transmission Control Protocol)/IP (Internet Protocol), a response confirmation time increases to decrease the throughput.